1. Field of the Invention
The present invention relates to a process for the manufacture of paper having a high smoothness, wherein the paper arriving from the paper machine is supplied on-line to a supercalender (multinip soft calendar), wherein the paper travels through a plurality of nips for achieving the desired and smoothness properties.
The present invention also relates to an apparatus for carrying out the process.
2. Description of the Related Art
Gloss and smoothness are characteristics of a paper web which not only influence the appearance of the paper web but also the possibilities of further processing of the paper web. In certain types of applications, gloss and/or smoothness values are desired which are to be reproducible as uniformly as possible.
Gloss and smoothness of the paper web are usually increased by conducting the paper web following the dry end of the paper machine into a smoothing unit which is composed of one or more nips, wherein the gloss and/or the smoothness of the paper web is increased by the pressure in the nip and the temperature of the rolls forming the nip. However, this makes it only possible to influence the gloss and smoothness of the paper web to a limited extent because, when the pressure in the nip is increased too much, the paper web is compressed too much and a volume loss of the paper web occurs. In this connection, there is the additional danger that the paper web loses stiffness. There are also limits with respect to the increase of the roll temperature because this increase requires a large amount of energy. For example, for achieving roll temperatures of 200xc2x0 C., significant quantities of energy must be supplied because the rolls are continuously cooled by the paper web travelling past the rolls.
Therefore, it has already been attempted, for example, in silicon papers, to influence the gloss and smoothness of the paper web by the moisture of the paper web. However, this has the disadvantage that after the treatment the supplied moisture has to be removed at least partially which requires additional process steps which, in turn, increase the time required for the treatment of the paper web and the apparatus is more complicated.
Basically, there are essentially two types of smoothing units. So-called supercalenders have a plurality of rolls arranged one above the other and nips provided between the rolls through which the paper web travels. The large number of nips produces a high degree of overlap or contact and a good distribution of the glazing work between pressure and temperature. Supercalenders are usually provided off-line, i.e., the paper web arriving from the paper machine is initially wound onto a reel-spool and is transferred together with the reel-spool to the supercalender, wherein the paper web travels through the supercalender at a significantly lower speed than the paper machine speed. The off-line installation has the advantage that the paper web can level out or equalize prior to entering the supercalender, so that the operation in the supercalender does not have to be carried out under the requirements of the paper machine which is influenced by many factors. However, the installation requirements are significantly higher. A supercalender classically has heated steel rolls, on the one hand, and paper rolls or rolls covered with cotton, on the other hand. More recently, also so-called multinip soft calendars are used in which the paper rolls are replaced by rolls having polymer covers. These rolls have a different elastic behavior than the paper rolls, so that it is possible to operate with a lower nip pressure.
Secondly, there are so-called machine calendars or soft calendars which can be connected on-line to a paper machine and through which, consequently, the paper web travels with the paper machine speed. However, machine calendars only have a small number of nips, so that the operation is carried out with higher pressure and temperature and, thus, the paper web is stressed to a greater extent. A significant disadvantage of the soft calendars is that not all types of paper can be upgraded to high qualities. In particular, it is not possible to manufacture highly compacted SC-A paper on-line in a soft calendar. It was possible recently to achieve the printing properties of a natural gravure paper supercalendared in 11 nips in a soft calendar with only four nips; however, this requires relatively high roll temperatures and compressive stresses in the nips. Also, these qualities can only be achieved in a range of speeds which corresponds to the glazing speed in the supercalender which is usual for this paper (see: Rothfuss, Ulrich: Inline- und Offline-Satinage von holzhaltigen, tiefdruckfxc3xa4higen Naturdruckpapieren in: Wochenblatt fxc3xcr Paperfabrikation 1993, No. 11/12, pages 457-466). Consequently, such qualities can only be achieved with the off-line installation of the soft calendar.
DE 43 01 023 C2 discloses in connection with machine calendars a process which provides to apply steam to the paper web immediately prior to the nip and to conduct the paper web through the nip before the temperature and moisture have equalized in the paper web. This not only results in moistening of the paper web, but a temperature increase is achieved simultaneously. The heat contained in the steam is transferred during the condensation onto the paper web, so that this measure produces a paper web which has at the surface thereof the necessary temperature and the necessary moisture. When this paper web is guided through the nip, the nip primarily influences the surface portion of the paper web, while the middle and bottom portions are influenced significantly less. Consequently, no changes worth mentioning occur in the thickness direction in the middle (and bottom) portions. The volume of the paper web is maintained to a greater extent, although the surface quality is significantly improved. However, the on-line manufacture of SC-A papers is not possible with machine calendars of this type.
Therefore, it is the primary object of the present invention to manufacture paper having a high smoothness on-line.
In accordance with the present invention, the paper web arriving from the paper machine and supplied on-line to a supercalender (multinip soft calendar) is moistened with spray mist and the spray mist is applied onto the paper web about 0.6 to 1.2 s, preferably 0.8 to 1 s, before the paper web travels through the first nip.
The moistening of paper webs by means of nozzle-type moistening units is basically known in the art. A spray head suitable for this purpose is known, for example, from DE 38 19 762 C2. However, since during spraying individual droplets are applied onto the paper web, the paper web is inevitably moistened unevenly. Therefore, this type of moistening has in the past only been used in the off-line treatment of paper webs, so that the moisture has sufficient time to distribute uniformly over the paper web. The on-line moistening has in the past been carried out by means of steam blow boxes, wherein by definition the formation of droplets in the spray mist is to be prevented. However, we have recognized that the selection of an appropriate distance between the spray mist moistening and the first nip of the supercalender makes possible a uniformly moistened paper web even during the on-line installation and on a paper web conveyed with the paper machine speed. It was found that a duration of 0.6 to 1.2 s, preferably 0.8 to 1 s, is sufficient for obtaining, in addition to a uniform moistening across its surface, a paper web that is uniformly moistened in the Z-direction, i.e., across its cross-section or thickness. Accordingly, the selection of the precise position of the moistening device depends significantly on the paper machine speed. At a usual paper machine speed of up to 1,400 m/min and a desired time of application of 0.7 s, the spatial distance of the moistening unit from the first nip is about 16 m. In paper machines with appropriate deflections of the paper web, this can be realized without problems.
In order to ensure that the moisture applied in the form of droplets onto the paper web is distributed evenly sufficiently quickly, a preferred embodiment of the present invention provides that the spray mist is applied in the form of a fine uniform mist with an average droplet size  less than 50 xcexcm, preferably xe2x89xa620 xcexcm. The formation of excessively large droplets would make it impossible to uniformly distribute the moisture in the paper web, i.e., in the Z-direction, which, in turn, would lead to a deterioration of the quality during the treatment in the nips.
Since especially in the case of a steam application immediately in front of the first nip, as proposed in DE 43 01 023 C2, the paper web is treated gently in the calendar, it is possible to supply the paper web to the calendar with a relatively high initial moisture. Accordingly, the present invention provides that the moisture content of the paper web is increased by the spray mist application by about 5 to 7%. Consequently, starting from an average moisture of 2 to 4% after leaving the paper machine, the paper web has after moistening a moisture of about 7 to 11%. The increased moisture content makes possible a better deformability of the paper web at the first nip of the supercalender.
In accordance with a further development of the present invention, a better penetration of the moisture in the Z-direction into the paper web is achieved by producing the spray mist with water having a reduced surface tension. Thus, in accordance with the invention, the spray mist is produced with warm water or tensides or the like are added to the spray mist water in order to reduce the surface tension.
In accordance with the invention, the droplet size is reduced by spraying the spray mist with air.
The present invention further provides that the moisture of the paper web is determined before and/or behind the moistening unit and moistening is controlled in dependence on the determined actual moisture values and the predetermined nominal values.
In accordance with a further development of the invention, the gloss and/or the smoothness of the paper web are measured and any gloss and/or smoothness differences over the width of the paper web are determined. The individual spray heads of the nozzle-type moistening unit can be controlled individually, so that the application of mist can be adjusted in the transverse direction of the paper web according to the determined gloss and/or smoothness differences.
In accordance with the invention, the upper and bottom sides of the paper web are moistened, so that the quantity of moisture to be applied to each individual side can be reduced accordingly. In addition, when moisture is applied to only one side, there may be problems with respect to the flatness of the paper web, i.e., the border areas may slightly bend.
Finally, in accordance with the present invention it is further provided that the paper web is moistened with steam immediately prior to the first nip of the supercalender and is guided through the first nip before the increased moisture of the surface resulting from the application of steam has dropped below a predetermined value in the range of 12% to 25%. As a result, as already described above, the influence of the nip on the paper web is essentially limited to the surface portions of the paper web and a gentler treatment of the paper web in the supercalender is made possible. By applying steam, it is additionally possible to compensate for flatness problems of the paper web, so that the paper web enters the supercalender properly in a flat state.
In accordance with the present invention, in an apparatus for the on-line manufacture of paper including a paper machine with a dry end and a supercalender (multinip soft calendar) with a plurality of nips through which the paper web travels following the dry end, a moisture application device with at least one nozzle-type moistening unit is arranged following the dry end of the paper machine for applying a spray mist on the paper web, wherein the distance of the moisture application device from the supercalender is such that the paper web travels through the first nip about 0.6 to 1.2 s, preferably 0.8 to 1 s, after the moistening of the paper web.
In accordance with the present invention, the moisture application device has at least one nozzle unit for the upper and the bottom side of the paper web.
In accordance with a preferred development of the invention, a suction means is provided at the nozzle unit which makes it possible to hold the mist within the system and to ensure a uniform distance of the paper web from the nozzles. The suction means produces a negative pressure which serves to compensate for the fact that the paper web is pressed by the spray mist application away from the nozzle unit.
In accordance with a further development of the present invention, especially the nozzles for moistening the upper side of the paper web are arranged horizontally; this is because a downward spraying direction could result in problems concerning the formation of droplets.
In accordance with a preferred feature, a plurality of spray heads are arranged next to each other in the nozzle unit. In accordance with the invention, these nozzle heads can be controlled individually or in groups. This makes it possible to compensate for any determined gloss and/or smoothness differences in the transverse direction of the paper web by appropriately controlling the spraying intensity of individual spray nozzles.
A steam delivery device is provided immediately prior to the first nip of the supercalender, so that the temperature increase and moisture increase of the paper effected by the steam application has not yet been equalized when the paper web travels through the nip.
In accordance with a further development of the invention, a steam discharge device each is arranged on both sides of the paper web in front of the first nip of the supercalender, so that the improvement of the gloss and smoothness properties takes place simultaneously on the upper side as well as on the bottom side of the paper web.
In accordance with a further development of the invention, a measuring frame is provided prior and/or behind the moisture application device, wherein the measuring frame determines the moisture of the paper web, and wherein the determined measurement values are utilized for controlling the moisture application device and/or the steam delivery device.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.